In circuit design of millimeter wave circuits, there is known a structure in which a microstrip line, which is converted from a high-frequency transmission line such as a microstrip line connected to a semiconductor device or a coplanar line connected to a semiconductor device, is connected (converted) to a laminated waveguide.
A connection structure disclosed in Japanese Unexamined Patent Publication JP-A 2000-252712 is a structure in which a strip conductor of a microstrip line is disposed so as to intersect a longitudinal axis of a slot of a laminated waveguide, and the microstrip line is coupled with the laminated waveguide via a slot formed in a ground conductor.
However, in the connection structure disclosed in JP-A 2000-252712, an electromagnetic coupling with the strip conductor is achieved via the slot. Therefore, in S-parameters, a frequency bandwidth where reflected waves (S11) have −20 dB or lower is 3.7 GHz, and thus, the fractional bandwidth is small at a desired design frequency of 76.5 GHz. Therefore, it cannot be said that reflected waves have excellent frequency characteristics.
A waveguide/microstrip line converter disclosed in JP-A 2002-208806 has a configuration such that a strip conductor of a microstrip line is electrically connected to an upper main conductor layer of the waveguide, a ground conductor of the microstrip line is electrically connected to a lower main conductor layer of the waveguide, and the strip conductor is formed into a tapered shape so as to achieve an impedance matching therebetween.
However, in the surface having such a tapered portion, as the width of the tapered portion becomes closer to the width of the waveguide, the electric field distribution of signals propagating through the microstrip line broadens in the width direction of the tapered portion and has a different distribution from the electric field distribution of the waveguide. Therefore, electromagnetic waves leak at a connection portion and a conversion loss associated with a mode conversion increases.